Doors are increasingly being manufactured from composite components containing fiberglass and thermoset polymers. Typical composite door assemblies include a pair of compression molded exterior skins (also referred to herein and generally known as “facings”). The door skins are mounted on a rectangular usually interior frame, frequently formed of wood, that separates and supports the skins in spaced relationship to one another. A hollow space between the skins and bounded by the door frame typically is filled with an insulating material, for example, cardboard, paper, fiberboard, or foam such as polyurethane.
Composite door assemblies provide several advantages over natural and steel doors. Composite door assemblies resist rot and corrosion experienced with solid wood and metal doors, respectively. The composite door assemblies also generally are better insulators than solid wood and metal doors. Because of material costs and manufacturing efficiencies, polymer composite door assemblies are considerably less expensive to manufacture than solid wood doors and can be designed to provide a reasonable facsimile of a wood grain on their outer surfaces.
A typical compression molding process used for making commercially available molded door skins involves placing a predetermined weight of sheet molding compound (SMC) charge layers containing a thermosetting polymeric material and fiberglass reinforcement on a lower mold half. An upper mold half is then advanced into engagement with the lower mold half to force the SMC material to fill and to conform to the shape of the mold cavity during compression. The mold halves are heated to a processing temperature, for example about 150° C., to facilitate flow of the resin and cause the thermosetting reaction, also known as curing. Once solidified, the molded door skins are removed from the mold press. Often, the mold dies have contours and embossing features that imprint depressions, grooves, patterns, texture and the like into the molded door skin. The imprinted features often are configured as one or more square or rectangular depressions simulating the perimeter of one or more panels. Alternatively, the facings may be flush. The design flexibility makes composite door assemblies useful for residential and commercial buildings, as well as other uses.
Typically, sheet molding compounds contain a thermosetting resin system such as an unsaturated polyester resin and an unsaturated co-curable reactive monomer, such as styrene. The sheet molding compounds also contain a reinforcing agent, such as glass fibers, often presented as chopped fiberglass and/or a thin fiber mat. Additives commonly combined with sheet molding compounds include catalysts, activating agents, thickening agents, stabilizers, and inert fillers such as calcium carbonate, talc, and wood particles.
The use of conventional compression molded door skins in geographical areas having high or low temperature seasonal climates can be problematic. For example, in areas such as in the northern United States and Canada, low winter temperatures can create a high thermal gradient between, on the one hand, the outside door skin exposed to environmental conditions and, on the other hand, the inside door skin warmed by the internal heating of its associated house or building. This temperature gradient can cause the door skins, particularly the outer door skin, to temporarily or permanently bow or warp. While warping and bowing distances smaller than ¼ inch may be tolerable, greater bowing of the door can violate building codes, blight the door's aesthetic appearance, and impair functionality of the door. For example, a warped door may be difficult to close in a sealing manner with respect to its exterior door frame, to which the doors is pivotally attached, usually by hinges.
One conventional solution for reducing bowing is to reinforce the frame of the composite door with steel rods or the like. In theory, the steel rods incorporated into the interior door frame should possess sufficient strength and stiffness to negate warping forces of the skin secured to the door frame. However, in practice it has been found that the steel rods do not prevent unacceptable bowing in extreme temperature climates. Further, steel reinforcement adds to the overall cost and weight of the door, making it more expensive and laborious to ship and install the door.